Get a few really nice batteries if you do not already have them.
For M4s, I recommend this
I would also get a madbull tightbore, systema bucking, SHS metal rack piston, metal bearing spring guide, new spring (depending on desired FPS),and an air-seal nozzle.
Some good things you can do to the weapon for cheap are:
-Reshim, regrease, reseal, adjust AOE, polish everything, wire to deans, and follow this guide
-Also, put in some sorbothane padding to increase the life of the V2 (and fillet the gearbox where the cylinder makes contact)
Good luck with your upgrades!